Before Newton, no one had heard of gravity, let alone the concept of a universal law. His masterwork, Philosophiae Naturalis Principia Mathematica, laid out his theory of gravity, which he described as an ever-present force, a tug that all objects exert on nearby objects. The more mass an object has, the stronger its tug, so increasing the distance between two objects weakens the attraction.
To put it in the terms that were likely conveyed to us at school, Newton's law of universal gravitation states that every mass attracts every other mass in the universe, and the gravitational force between two bodies is proportional to the product of their masses, and inversely proportional to the square of the distance between them.
Basically, that means that the force of gravity is always attractive, works instantaneously at a distance, and has an infinite range. Most importantly, it affects everything with mass - and has nothing to do with an object's charge or chemical composition.
In 1915, Einstein turned this theory on its head, hypothesising that there was an agent that caused gravity. In this theory, the agent wasn’t simply a force. According to Einstein’s theory of general relativity, gravity is a natural consequence of a mass’s influence on space.
Einstein agreed with Newton that space had dimension that included width, length, and height, but he believed that space was affected by the objects in it, which was something Newton’s theory rejected. This is where the term ‘fourth dimension’ comes from - Einstein believed gravity was a distortion in the shape of space-time, which has come to be referred to as the fourth dimension.
Since then, scientists have come to agree with Einstein’s views, but have only been able to theoretically point to the veracity of his theory compared to Newton’s. Now, it seems that even Einstein wasn’t quite right.
A team of researchers that has been studying the star known as S0-2, or Sagittarius A, which is the body closest to the supermassive black hole at the centre of the galaxy. Following the orbit of S0-2 for more than two decades, the scientists have been able to map its trajectory in three dimensions, and the behaviour of the star backs up Einstein, and refutes Newton.
However, Andrea Ghez from the University of California, who led the team, says that Einstein’s theory is “definitely showing vulnerability”. “It cannot fully explain gravity inside a black hole, and at some point, we will need to move beyond Einstein’s theory to a more comprehensive theory of gravity that explains what a black hole is,” she says.
Knowing the orientation of the orbit of the star with respect to the black hole enabled the team to determine the star’s radial velocity and properly interpret subtle changes in SO-2’s light. Non-relativistic theories of gravity do not predict any changes to the light leaving the star as it orbits the black hole, speeding up as it passes close by and then slowing down as it moves away. The scientists were able to precisely measure changes in wavelength, and colour, as the star went through its orbit, and they found it matched Einstein’s predictions.
The team found that S0-2 underwent gravitational redshift as it passed near the black hole. Gravitational redshift is a phenomenon caused by gravity over distance, similar to how the Doppler effect changes characteristics of a wave in motion relative to its observer.
The end result is that these observations tested the theory of general relativity, and found that it held up. “We asked how gravity behaves near a supermassive black hole and whether Einstein’s theory is telling us the full story. Seeing stars go through their complete orbit provides the first opportunity to test fundamental physics using the motions of these stars. Einstein’s right, at least for now. We can absolutely rule out Newton’s law of gravity,” Ghez says.
The challenge for scientists now is to see if – and where – they can poke definitive holes in Einstein’s definition of gravity, and bring us to a better understanding of how the universe works.